Crimp can be defined as the non-linearity in fiber. For most of the man-made fibers employed in carpet manufacture, a non-woven battings, the crimp or bend in the fiber is induced by thermal/mechanical techniques. It can also be thought of as the difference between the non-linear (crimped) fiber and the straightened fiber (fiber extended). A crimp is important in carpet fibers and non-woven applications because it provides bulk to the yarn by preventing two fibers from laying parallel to one another. As a result, the carpet tuft will have greater covering power, appear softer, and give better resistance to wear and abrasion, among other benefits.
Crimp is also useful in the processing of staple fibers. Crimp is particularly useful in the processing of high modulus fibers which are difficult to work with because of slipperiness.
It is standard practice to crimp staple fibers such as polyester, acrylic, and nylon with steam and a stuffing box to provide pressure to result in a fiber which has waves or kinks in it. This crimp makes the fiber process better in nonwoven textile processes such as carding and permits the staple fiber to form high loft (low density) battings. This crimp is necessary for efficient processing of these fibers to spun yarns since the crimp is again necessary for efficient carding necessary for many forms of yarn making. The crimp also provides a means of further entangling the fibers and making a stronger yarn. From an overall textile point of view, crimp is important. However, textile fibers are thermoplastic and therefore the crimp comes out if the fibers are heated above their glass transition point. For instance, crimped continuous tow PAN fiber(acrylic) when heated to 210.degree.-250.degree. C. while undergoing preoxidation for the manufacture of OPF (oxidized polyacrylonitrile fiber) totally loses its crimp while being pulled through the oven.
Thermoset fibers such as carbon fibers and related fibers although relatively uncommon do not crimp since they do not soften or melt. Heat, steam, and pressure do not create crimp in carbon fiber, carbonaceous fibers (&gt;65% carbon) or any other thermoset fiber.
Crimp in the stuffer box is achieved by passing yarn(s) or tow(s) into a uniformly heated chamber which is at the temperature required to heat set the fibers in their crimped or non-linear configuration. Steam is usually used to assist and lubricate. As the yarns are forced into the chamber by feed rolls, it pushes against yarn which is already in the chamber, thereby causing the filaments to bend and buckle (crimp).
A weight tube fitted into the top of the stuffer box governs the flow and quantity of yarn into the stuffer box. The frequency (crimps per inch) and the crimp amplitude of the fibers are controlled by regulating the speed of the feed rolls to that of the take up rolls as well as the weight of the tube. Crimp setting by these techniques can be done for single filaments or on multiple ends (tow) using the spunize technique. The crimps are generally characterized by numerous sharp bends.
U.S. Pat. No. 4,868,038 of McCullough et al, which is herein incorporated by reference, discloses one method for preparing novel non-linear carbonaceous fibers having physical characteristics resulting from heat treating stabilized polymeric fibers in the form of a knitted fabric. There is described a process wherein the fabric is substantially irreversible heat set under conditions free of non-uniform stress and tension. In order to obtain fibers which are non-linear, it is necessary to deknit the continuous fibers and then chopping to the desired length. Knitting and then deknitting the fabric to obtain non-linear carbonaceous fibers increases the cost in producing the fibers. This crimp is permanent and will not disappear on subsequent heat treatments.
Robinson U.S. Pat. No. 2,245,874, discloses a method for forming curled fiber material by passing fibers over rollers under conditions to bend and stretch the fibers beyond elastic limits. Such a process cannot be used to produce the non-linear fibers of the invention. This crimp can be removed by heating the fiber crimped up to the glass transition point.
Hemmi U.S. Pat. No. 2,623,266 discloses the mechanical preparation of sinusoid or spiraloid crimped fibers. The fibers are heated and passed through a series of bars which impart a meander-like crimp. However, the fibers are formed in a crimped and stretched state. It is desirable to provide a relatively inexpensive and simple method for producing non-linear fibers and tows.
It is further desirable to provide a method for producing non-linear fibers which does not require the prior formation of a fabric.
It is also desirable to prepare a non-linear carbonaceous fibers without performing a knit-deknit operation.